Over the past fifteen to twenty years, the potential dangers of hazardous waste, and in particular, heavy metals such as lead, arsenic, chromium, copper and cadmium, have been the subject of community pressure, public awareness and ever stricter regulatory control in order to reduce or eliminate the dangers to people directly and to the surrounding environment.
The leaching of heavy metals, and lead particularly, into ground water is a grave concern because of the danger that the drinking water supplies and the environment will become contaminated.
Solid wastes are classified as hazardous by the United States Environmental Protection Agency (U.S. EPA) pursuant to 40 C.F.R., Part 261. Any solid waste can be defined as hazardous either because it is "listed" in 40 C.F.R., Part 261 Subpart D or because it exhibits one or more of the characteristics of a hazardous waste as defined at Part 261, Subpart C. These characteristics are: (1) ignitability, (2) corrosivity, (3) reactivity, and (4) toxicity.
40 C.F.R., Part 261.24(a), contains a list of contaminants and their associated maximum allowable concentrations. If a contaminant, such as lead, exceeds its maximum allowable concentration, when tested using the TCLP analysis as specified at 40 C.F.R. Part 261 Appendix 2, then the material is classified as hazardous. Waste containing leachable lead (Pb) is currently classified as hazardous waste due to the toxicity characteristic, if the level of lead extracted in a TCLP analysis is above 5.0 milligrams per liter (mg/L) or parts per millions (ppm).
Additionally, U.S. EPA land disposal restrictions prohibit the land disposal of solid wastes which leach in excess of these maximum allowable concentrations upon performance of the TCLP analysis. The land disposal regulations require that these wastes are treated until the heavy metals do not leach from the solid waste at levels above the maximum allowable concentrations prior to placement in a surface impoundment, waste pile, landfill or other land disposal unit as defined in 40 C.F.R. 260.10.
In addition, various states also have promulgated regulations which place special disposal requirements on solid waste which do not leach at concentrations that exceed the TC criteria but do leach above certain other criteria. For example, the State of Texas classifies solid wastes which leach lead between 1.5 mg/L and 5.0 mg/L under the TCLP analysis as a non-hazardous, Class 1 solid waste. Special disposal requirements are placed on these wastes which are more costly than if the TCLP results are less than the 1.5 mg/L. Therefore, the goal of solid waste treatment is often to obtain leaching results much lower than the hazardous criteria of 5.0 mg/L.
The TCLP test is designed to simulate a worst case leaching situation, that is leaching conditions which would typically be found in the interior of an actively degrading municipal landfill. Such landfills normally are slightly acidic with a pH of approximately 5.+-.0.5. Additionally, the leaching medium is normally heavily buffered by volatile organic acids, for example acetic acid produced by the facultative anaerobic degradation of organic matter. Currently, Applicant's pending patent application Ser. No. 08/726,622, filed Oct. 7, 1996 and which issued as U.S. Pat. No. 5,674,176 on Oct. 7, 1997, covers treatment to reduce the toxicity of a waste as measured by the TCLP test.
In addition to the TCLP leachate test requirements, structural design and regulatory requirements often place physical criteria on treated waste. The U.S. EPA has adopted 50 pounds/square inch, measured as unconfined compressive strength, as its general guidance for structural criteria to support a landfill cap.
Importantly, if the treated waste is placed in a saturated or potentially saturated condition, a permeability specification may also be placed on the treatment design. For example, the State of Florida has imposed a maximum permeability criteria of 1.times.10.sup.-6 cm/s on waste disposed of in subsurface saturated conditions. If the metal containing waste has a significant amount of sand or organic matter, then additives will be needed to obtain the required structural integrity. Under actual disposal conditions, reductions in permeability will reduce percolation of surface and ground water into water tables carrying leachable heavy metals, and in particular lead.
The present invention provides a method of reducing the permeability of the solid waste and, thus, reducing the leachability of certain heavy metals.
Unlike the present invention, prior art additives and mixtures have focused only on reducing the leachability of the metals without regard to permeability standards or characteristics. The present invention reduces the permeability of the solid waste and thereby also reduces the leachability of heavy metals, and in particular lead.
U.S. Pat. No. 5,591,116 which was assigned to the assignee of the present application and which is hereby incorporated by reference, teaches the combination of additives and Portland Cement to reduce the leachability of heavy metals and in particular lead.
U.S. Pat. No. 5,202,033 describes an in-situ method for decreasing heavy metal leaching from solid waste using a combination of solid waste additives and additional pH controlling agents from the source of phosphate, carbonate, and sulfates.
U.S. Pat. No. 5,037,479 discloses a method for treating highly hazardous waste containing unacceptable levels of leachable metals such as lead by mixing the solid waste with a buffering agent selected from the group consisting of magnesium oxide, magnesium hydroxide, reactive calcium carbonates and reactive magnesium carbonates with an additional agent which is either an acid or salt containing an anion from the group consisting of Triple Superphosphate (TSP), ammonium phosphate, diammonium phosphate, phosphoric acid, boric acid and metallic iron.
U.S. Pat. No. 4,889,640 discloses a method and mixture for treating hazardous waste, including lead, by mixing the solid waste with an agent selected from the group consisting of reactive calcium carbonate, reactive magnesium carbonate and reactive calcium magnesium carbonate.
U.S. Pat. No. 4,652,381 discloses a process for treating industrial waste water contaminated with battery plant waste, such as sulfuric acid and heavy metals by treating the waste water with calcium carbonate, calcium sulfate, calcium hydroxide to complete a separation of the heavy metals. However, this is not for use in a landfill situation.
U.S. Pat. No. 5,536,899 discloses a method for reducing the leaching of lead from lead bearing waste by contacting the waste with a water soluble phosphate and a complexing agent, whereby a lead product is formed that is less soluble than the lead originally in the waste, thereby reducing the leachability of lead from the waste.
U.S. Pat. No. 5,527,982 discloses a method for treating lead bearing materials or lead waste which chemically converts leachable lead into a non-leachable form by mixing the material with lime, gypsum and/or phosphoric acid.
U.S. Pat. No. 5,193,936 discloses a two step treatment process for lead-toxic wastes to fixate and stabilize leachable lead contained therein, which employs the use of a sulphate compound, such as gypsum, in a first step, and a phosphate reagent, such as phosphoric acid, in a second step.
Unlike the present invention, however, none of the prior art solutions were able to utilize the waste itself in combination with a single additive or multiple additives to reduce the permeability of the solid waste and thereby further reduce its leachability and mobility.